MCAT Biology : Developmental Stages and Events

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #33 : Reproductive System And Development

What is embryonic cleavage?

Possible Answers:

DNA replication

Division of cells in the early embryo

Division of the zygote into germ layers

Growth of the cell cytoplasm and organelles

Correct answer:

Division of cells in the early embryo

Explanation:

Embryonic cleavage is the division of cells without cell growth. Once an ovum is fertilized by a sperm cell, it is called a zygote. The zygote undergoes multiple rapid cell cycles (rounds of mitosis) without significant growth, producing a dense cluster of cells that is the same size as the original zygote. This process is called cleavage.

Example Question #34 : Reproductive System And Development

Which stage of embryogenesis if the first to allow cell growth?

Possible Answers:

Blastula

Zygote

Morula

Gastrula

Correct answer:

Blastula

Explanation:

Embryogenesis is the process by which an embryo forms, and begins with fertilization of an ovum by a sperm cell. Once an ovum is fertilized, it is referred to as a zygote. The zygote undergoes rapid mitotic divisions without any significant cell growth, a process called cleavage. After four cell divisions occur, a ball of sixteen cells is produced, which is called the morula. After the seventh cleavage, a ball of 128 cells called the blastula is produced. The blastula allows the addition of the G1 and G2 phases into the cell cycle, permitting the cells to grow in size before dividing. The blastula develops an inner cell mass and trophoblast, and becomes implanted in the uterine lining. A gastrula is formed when cells migrate to the interior of the blastula and form three germ layers: the endoderm, mesoderm, and ectoderm. A neurula forms after differentiation of the ectoderm forms the primitive central nervous structures.

Example Question #35 : Reproductive System And Development

Which of the given embryonic events occurs first?

Possible Answers:

Gastrulation

Implantation

Cleavage

Neurulation

Correct answer:

Cleavage

Explanation:

The correct order of the early development is: cleavage, implantation, gastrulation, neurulation.

Cleavage occurs when zygote undergoes many mitotic divisions without cellular growth, becoming a morula and then a blastula. Only after becoming a blastula will the cells begin to grow in size.

Implantation occurs during the blastula stage when the embryo becomes implanted in the uterine lining.

Gastrulation occurs when the three germ layers (endoderm, mesoderm and ectoderm) arise from the blastula. At this stage, the embryo is considered a gastrula.

Neurulation occurs when the germ layers develop a primitive nervous system, transitioning the embryo from a gastrula to a neurula.

Example Question #36 : Reproductive System And Development

Which of the following is false regarding cleavage?

Possible Answers:

Cleavage occurs regardless of the sex of the fetus

Cleavage happens rapidly after fertilization, but does not continue indefinitely

It leads to multiple tiny cells, whose volumes all add up to the original cell size

It results in multiple cells that are each the same size as the original cell

It is the first step of embryogenesis after fertilization

Correct answer:

It results in multiple cells that are each the same size as the original cell

Explanation:

Cleavage occurs immediately after fertilization. It is a rapid process that increases cell number without increase cell size. Essentially, cells divide without any G1 or G2 phase of the cell cycle. The result is several very small cells that take up the same volume as the original singular zygote.

Cleavage generates the morula and, later, the blastula during development. At the blastula stage, the cells develop the G1 and G2 phases of the cell cycle and begin more comprehensive development. At this stage, the embryo begins to differentiate the inner cell mass and trophoblast regions.

Example Question #37 : Reproductive System And Development

During what stage of embryogenesis does implantation of the embryo occur?

Possible Answers:

Gastrula

Blastula

Morula

Zygote

Correct answer:

Blastula

Explanation:

Implantation of the embryo occurs during the blastula stage.

The zygote is initially formed by the process of fertilization in the fallopian tube. As the zygote travels within the tube, it begins to undergo cleavage, developing into a morula and eventually a blastula. By this point, it has entered the uterus and can become implanted in the uterine lining. The blastula then begins to differentiate, developing the three germ layers, and becoming a gastrula via the process of gastrulation.

Example Question #91 : Animal Biology

The inner cell mass (or embryoblast) of the blastocyst gives rise to all of the following layers except __________.

Possible Answers:

the mesoderm

the primitive endoderm

the trophoblast

the epiblast

Correct answer:

the trophoblast

Explanation:

The blastula, or blastocyst, is made up of blastomere cells and a fluid-filled cavity called the blastocoel. 

There are two regions of the blastocyst: the inner cell mass and the trophoblast. The inner cell mass gives rise to the primitive endoderm and the epiblast, which later gives rise to the three germ layers during gastrulation. The trophoblast is the layer of cells forming the outer ring of the blastocyst. It secretes factors to make the blastocoel and is kept separate from the inner cell mass. All fetal structures eventually develop from the inner cell mass, while the trophoblast helps maintain the fetal environment and placenta.

Example Question #38 : Reproductive System And Development

Which of the following is not a property of the trophoblast?

Possible Answers:

Cells that form the outer layer of the blastula 

Gives rise to the embryo

Gives rise to the placenta

Secretes factors to make the blastocoel

Correct answer:

Gives rise to the embryo

Explanation:

The blastula consists of a hollow sphere of cells (blastomeres) surrounding an inner fluid cavity called the blastocoel. There are two regions of the blastula: the inner cell mass (embryoblast) and the trophoblast. The inner cell mass gives rise to the primitive endoderm and epiblast, serving as the site of the developing embryo. The trophoblast cells form the outer ring of the blastocyst and combine with the maternal endometrium to form the placenta.

The inner cell mass, or embryoblast, gives rise to the cells that become the fetus.

Example Question #39 : Reproductive System And Development

What is a blastula?

Possible Answers:

Sphere of cells surrounding a blastocel

Structure that gives rise to embryonic cleavage

Precursor to the morula

Solid ball of cells within the zona pellucida

Correct answer:

Sphere of cells surrounding a blastocel

Explanation:

A blastula is a sphere of cells (blastomeres) surrounding a fluid blastocoele. The blastula stage of embryogenesis begins with the appearance of the blastocoel. The blastocoel is a fluid filled cavity that contains amino acids, proteins, growth factors, and other components neccessary for cellular differentiation. The blastocoel allows blastomeres to move during gastrulation.

A morula is a solid ball of cells within the zona pellucida and is a precursor to the blastula.

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